I have been studying James 5:12. James is a difficult book to outline because much of the book seems disjointed and more like a collection of sayings. Indeed, at times, scholars have called it the Proverbs of the New Testament. However, there are many key motifs throughout the book of James (suffering, correct speech, true faith, etc.). These themes appear often throughout the book. In the case of James 5:12, James is returning to the theme of correct speech (cf. 3:1–12 and 4:11–12).
The first decision one needs to make about this verse is its relationship to the rest of the epistle. Some claim it is a continuation of the previous section concerning persecution. Christians were tempted to take oaths to declare their truthfulness in the midst of persecution. However, this seems to be a difficult connection to establish. It is probably better to see James’ words "above all things" to be a formal introduction to the conclusion of his epistle. Much like Paul’s word “finally” ( 2 Cor 13:11). If that is the case then James is beginning the conclusion of his epistle and is giving his last few exhortations to his readers. This is what leads him back to his correct speech motif.
The other thorny issue in this passage is the application of oath taking today. Clearly, this passage condemns the first century Jewish practice of making certain kinds of oaths binding and other kinds non-binding. Whether it deals with courtroom oaths is much debated. I will state my opinion in that regard in the comments. However, there is an undeniable application to this passage.
This passage clearly tells us that when we say "yes" it should mean yes and when we say "no" it should mean no. In other words, no one should ever have a reason to doubt the truthfulness of our statements. As Christians we should be known as people of truth and that in effect makes oaths unnecessary.
This is an important issue for Christians. Unfortunately, too many Christians play fast and loose with the truth. The reputation of many Christians in regard to truth is not good and especially not unquestionable. We need to be committed to being truthful people. Truthful even when it costs us something.